My Galaxy S5 did just fine underwater too (of course after dropping it several times over the last two years it finally cracked, so I don't think it's very water proof anymore. haha). I just got the S7 Edge, and I was hoping it would be the same underwater as my S5 was. Glad to see it is!
I just shot underwater for the first time today with my S7 Edge. We were at an outdoor swimming pool that was 5 feet deep and it worked great! A few things I learned, however:
1) The touch screen doesn't work underwater, so you need alternative ways of snapping a photo or starting and stopping video. For photos, you can set the Volume Buttons to take a picture, so that solves that problem. For video, I just started the video before going under and that worked for the most part, except sometimes the video would just stop recording while underwater OR immediately after surfacing. Not sure if I was hitting something, or if it's just an anomaly?
2) Sometimes you have to wipe or brush the water off of the lens in the back camera so that your pics/videos aren't foggy or smeared. Not a big deal, but something you want to check between shots.
3) The screen was very difficult to see in the bright sunlight of the outdoor pool, so I had a very hard time determining the framing of the shots.
4) After we finished, I dried the phone off with a towel. Playing back audio was very distorted and muffled. Also, even though it looked like my charging port was dry, the phone would not charge for a while (got a message saying moisture was detected). Both of these problems cleared up after an hour or so. Audio is back to normal and it charged just fine again.
Overall, I am VERY impressed! Though I would use it as a dedicated underwater camera, I will definitely use it again at the pool with my family. Nice job Samsung!